For the purpose of increasing the optical band gap of a buffer layer in the case of fabricating an amorphous-silicon solar cell of the single type, and preferably for the purpose of increasing the optical band gaps of the buffer layer and i-layer of a top cell in the case of fabricating an amorphous-silicon solar cell of the integrated type, use is being made of a hydrogenated amorphous silicon film and a hydrogenated amorphous silicon carbide film, both produced by a method in which SiH.sub.4 as a major source gas is diluted with hydrogen. In addition, an a-SiO:H (amorphous-SiO:H) film produced from hydrogen-diluted SiH.sub.4 and CO.sub.2 gases has come to be used recently for that purpose. In particular, it has been reported that such an a-SiO:H film produced from an SiH.sub.4 -CO.sub.2 system by the hydrogen dilution method attained a photoconductivity higher by about one order of magnitude than that of a conventional a-SiC:H film produced from an SiH.sub.4 -CH.sub.4 system by the hydrogen dilution method and having the same optical band gap, showing that the a-SiO:H film had higher quality than the conventional a-SiC:H film (The 22nd IEEE Photovoltaic Specialists Conference, pp. 1296-1301 ).
An a-Si:H film, a-SiC:H film, a-SiO:H film, and the like are used as buffer layers and i-layers in amorphous-silicon solar cells of the single type and integrated type. In particular, for use as the buffer layer and i-layer in the top cell of an amorphous-silicon solar cell of the integrated type, an amorphous silicon thin film is regarded as promising because it is of high quality and can be made to have a wider band gap. In other words, there is a desire for an amorphous silicon thin film having a higher conductivity and a wider optical band gap than the conventional amorphous silicon thin films obtained by decomposing a mixed gas comprising a highly hydrogen-diluted SiH.sub.4 gas and either CO.sub.2 or CH.sub.4 by the plasma CVD (chemical vapor deposition) method.